1. Field of the Invention
This invention relates to an active matrix substrate, a method of manufacturing the same, and a display device, such as a liquid-crystal display using an active matrix substrate or an EL (electroluminescence) display.
2. Description of the Related Art
Liquid-crystal display devices have the advantages of consuming less power and providing very detailed images and therefore are widely used in notebook personal computers, thin-model television sets, and the like.
Most of the liquid-crystal display devices presently used are formed on glass substrates and therefore tend to break easily. Thus, they need to be improved in this respect. At the same time, lighter display devices are required from the viewpoint of weight.
In addition, there have been strong demands for flexible liquid-crystal display devices capable of being curved freely like paper or being folded. To satisfy the demands, liquid-crystal display devices using light, flexible substrates superior in resistance to impact, such as made from plastic, have been proposed.
It is desirable that these display devices be capable of displaying sufficiently beautiful moving pictures. To do this, it is necessary to use an active matrix substrate where thin-film active elements, such as thin-film transistors, are integrated. That is, it is necessary to realize an active matrix substrate where a thin-film active element array is formed on a plastic substrate.
To form thin-film transistors using amorphous silicon or polysilicon currently widely used, high-temperature processes in the range of about 350° C. to 600° C. are essential and therefore it is difficult to form thin-film transistors on a plastic substrate capable of resisting temperatures up to about 200° C.
To solve this problem, the following technique has been proposed: in the technique, after thin-film transistors are formed on a highly heat-resistant substrate, such as a glass substrate, they are transferred onto a plastic substrate, thereby forming a thin-film transistor array on the plastic substrate.
With this method, thin-film transistors can be formed by conventional high-temperature processes, so that thin-film transistors with characteristics as good as those of conventional equivalents can be arranged on a plastic substrate. In this method, however, the cost for the transfer process is newly added as compared with the prior art, causing the problem of increasing the cost.
To solve this problem, the following method has been proposed: after a thin-film transistor substrate (or element formation substrate) on which thin-film transistors are formed very densely is formed on a highly heat-resistant substrate, such as glass, part of the thin-film transistors are transferred from the thin-film transistor substrate onto a plurality of plastic substrates (final substrates) sequentially, thereby forming a plurality of transistor arrays. In this case, the technique for selecting only the transistors to be transferred from a large number of thin-film transistors and transferring the selected ones is needed.
In the Jpn. Pat. Appln. KOKAI Publication No. 11-142878 (hereinafter, referred to as the prior art), a transfer destination substrate to which adhesive resin, such as acrylate-based UV cured resin or UV cured epoxy resin, is applied is laminated with a thin-film transistor substrate formed very densely beforehand on a UV peel resin, and ultraviolet rays are projected only onto the thin-film transistors selected with a photo mask, thereby selectively transferring the thin-film transistors.
In the prior art, the adhesive resin has adhesion only when only the ultraviolet-rays-exposed area is half-cured at a result of the projection of ultraviolet rays. That is, a transfer destination substrate that has adhesion when only the ultraviolet-rays-exposed area of the uniformly formed adhesive resin is half-cured has been proposed. The cured area on which ultraviolet rays are not projected is removed after the selective transfer.
Furthermore, another transfer destination substrate has been proposed which has concave sections in it and adhesive resin applied only to the concave sections, not uniformly, thereby localizing adhesion. Then, the thin-film transistors temporarily bonded onto a UV peel resin are transferred to a substrate having the localized adhesive layer. The UV peel resin has peeling quality when being exposed to ultraviolet rays.
However, the prior art has the following problems.
In the method of half-curing a part of the adhesive resin layer to be cured and thereby causing only the half-cured section to have adhesion, since the adhesive sections and the non-adhesive sections are located in the sama plane at the time of transfer, not only the thin-film transistors to be transferred are bonded to the half-cured sections of the adhesive resin layer, but also the thin-film transistors not to be transferred are pressed against the cured sections of the adhesive resin layer. As a result, there is a possibility that the thin-film transistors will be bonded to not only the half-cured sections but also the cured sections and therefore many faults will take place in the transfer.
Furthermore, in the prior art, when the cured sections of the adhesive resin layer have been removed after the selective transfer, the interconnection lines of the thin-film transistors are formed. Since the steps formed at the ends of the adhesive resin layer are almost vertical, the interconnection line breakage rate can increase because of so-called step breakages.
Moreover, in the method of applying adhesive to the concave sections, the non-adhesive face around the adhesive face is flush with the adhesive face. When the thin-film transistors to be transferred come into contact with the adhesive resin, the thin-film transistors not to be transferred are pressed against the transfer destination substrate. In addition, this method causes another problem: adhesive resin leaks from the concave sections and adheres to the surrounding thin-film transistors. As a result, some of the unselected thin-film transistors might be transferred. This makes it difficult to reduce the cost.
Therefore, there has been a need to realize an active matrix substrate which is formed at low cost on a less heat-resistant substrate, such as a plastic substrate, by a selective transfer method and which is as good in performance as a conventional equivalent, a method of producing the active matrix substrate, and a display device.